Modulation system



March 13, 1934. B. G. LAKE MODULATION SYSTEM Filed April 18, 1931 Emmi I I Fig l wue/wbm 50/3700 G ZHAE Patented 13, 1934 4 Claims.

(Granted under the act of March 3, 1833, as amended April 30, 1928; 370 C. G. 757) This invention relates to wave transmission circuits and has for its principal object to provide a method and apparatus for obtaining substantially perfect modulation of a carrier wave with a relatively srnall amount of input power.

Another object of this invention is to provide a modulation system in which the output of a vacuum tube oscillator or the output of a vacuum tube amplifier excited in any manner may be modulated within wide limits.

A further object is to provide a frequency changer associated with receiving circuits and to obtain substantially perfect modulation of the new frequency.

With the above and other objects in view, the invention consists in the construction, combination and arrangement of parts as will be hereinafter more fully described.

These objects are accomplished by controlling the effective cathode potential of the oscillator (whose output is modulated) in accordance with the modulating current input, and at the same time maintaining constant the potential diiference between the control electrode and anode.

The modulation system herein disclosed may be used wherever it is desired to modulate a high frequency carrier wave with a low frequency, as, for instance, in the transmission of sound by radio, or in the reception of radio signals in which case a modulated wave of one frequency may be detected and the result used to modulate a differen frequency either for re-broadcasting or for converting the received signal into a frequency that may be more efficiently amplified.

Reference is to be had to the accompanying drawing forming a part of this specification, in which Figs. 1 and 2 show diagrammatically circuits embodying the invention.

Referring to Fig. 1, four vacuum tubes are used; a radio frequency amplifier V1, a regenerative detector V2, an audio frequency amplifier and modulator V3, and an oscillator (to be modulated) V4. The anode or plate of tube V: is connected through a resistance R to the positive electrode of battery B and directly to the cathode or filament of oscillator tube V4. The anode or plate of oscillator tube V4 is connected through a coupling coil L1, to the positive electrode of battery B.

The oscillator circuit consists of coils L1, L2 and condensers C1, C2 and C3 connected as shown in Fig. 1. LzCi is permanently tuned to the desired frequency within the broadcast band (500 to 1500 k. c.) and operates at all times at the same frequency. The feedback coil L1 and bypass condenser C3 sustain oscillations in the oscillating circuit L2C1. Condenser C2 isolates the grid of V4 from the filament but serves as a radio frequency grid return or bypass to filament. Condensers C2 and C3 are of about .00025 in. f. capacity or less in order that they may offer low impedance to radio frequency currents but high impedance to the audio or modulating frequencies. These capacities may, however, be changed as the exigencies of the case may require.

Three batteries are used. The filament battery A supplies current for the filaments of V1, V2 and V3. The filament battery D supplies current for the filament of V4. Plate battery B of about 45 volts supplies plate current for all tubes. This voltage may, however, be changed with difierent tubes or as the nature of the case may require.

It will be noted from Fig. i, that the output or plate load of V3 consists of two parallel circuits, the plate circuit of V4 and the variable resistance R, i. e., the resistance R is in series with the plate circuit of V3 but is in parallel with the plate circuit of V4. Therefore, the plate current of V3 is divided between the plate circuit of V4 and resistance R.

The plate circuits of V4 and V3 are in series with the plate battery B, which, for example, may be 45 volts.

The grid return of V4 is connected to a point of suitable fixed potential in the plate battery B.

In order that the plate battery B will not be shor -circuited through a common filament connection, a separate filament battery D is provided for oscillator V4.

The operation of the circuit is as follows: With filament and plate batteries connected as shown, the coil L1 is adjusted for maximum feedback and the grid of V is connected to point 3 on the plate battery which is, for example, at about 28 volts positive. The value of resistance R is then reduced to the lowest value at which the tube will oscillate. Under these conditions V4 is oscillating at a frequency determined by the values of L201, with a plate voltage equal to the voltage drop across R, and with a grid bias voltage determined by the point at which its grid lead is connected to battery B. This is made about one volt negative bias with respect to the filament. Therefore, the following conditions exist (disregarding radio frequency voltages due to oscillation, which need not be considered) the plate of V4 is at a fixed positive voltage of 45; the grid of V4 is at a fixed positive voltage or" about 28 with respect to ground potential; the filament of V4 is about one volt positive with respect to the grid; the potential of the filament with respect to the grid and plate is determined by the voltage drop across R. For normal operating conditions, I prefer to make resistance of R about one-third the resistance of the plate circuit of V4; the current through R would therefore be about three times the current through the plate circuit of V4.

Now assume a negative signal voltage to be applied to the grid of V3. This negative voltage decreases the plate current of V3 and hence the sum of the currents through R and V; by proportional amounts. However, the plate resistance of V4 increases with a decrease of plate current and a large percentage of the current change takes place in the plate circuit of V4 and very little change takes place in current through R as the resistance of R remains constant. In addition to this effect, it should be noted that if the current through R does decrease, then the voltage drop through R will be less, which in turn decreases the effective plate voltage on V4. This in turn places a higher negative bias on V4 because its grid voltage is fixed and the potential of the filament of V4 becomes more positive, which is equivalent to making its grid more negative. Thus any reduction of current through R results in reducing the eifective plate voltage on V4 and increases its negative bias and vice versa. Both of these effects change the plate resistance.

The current through R remaining practically at a constant value regardless of plate current changes in V3, and with V4 carrying only onethird the current of V3, it will be seen that a 25% change of current in V3 results in a 75% change of current in V4. Hence, small audio signal voltages on the grid of V3 result in a relatively high percentage of modulation of V4, which is desired.

It is important to note that the modulator tube V3 is operating with a constant plate voltage because, when there is a constant current through resistance R, the effective plate voltage of the modulator is equal to the plate battery voltage less the voltage drop through R. Therefore the modulator tube is working on its static characteristic curve and not on its dynamic characteristic curve determined by external plate circuit resistance. The grid bias and input of the modulator must be of the proper value to permit it to operate on its static characteristic curve Without appreciable distortion.

The amplitude of the modulated radio frequency output of V4 is relatively small and may be applied to the input of a broadcast receiver or other radio frequency amplifier for further amplification.

One method of applying the modulated radio frequency output of V4 to a broadcast receiver or other radio frequency amplifier is shown in Fig. 1, where L3 is a pick-up coil coupled to the oscillator inductance and directly connected to the input terminals of a broadcast receiver or other radio frequency amplifier M.

It should be understood that the radio apparatus described in the foregoing in connection with this invention is used only to permit a clearer description of the operation of the device. Other typesof tubes may be used such as those using alternating current to heat the filament or electron emitter. Also, the plate battery may be replaced by any other suitable source of direct current and the L. C. circuit of the oscillator may be replaced by a suitable piezo electric crystal device.

In connection with the use of A. C. tubes of the commonly called heater type, it should be noted that a separate source of heater current is not required for the oscillator as there is no electrical connection in this type of tube between the filament proper and the electron emitter.

The application of this system of modulation as applied to radio telephone transmission is shown in Fig. 2. The operation of the modulation system is the same as that described in Fig. 1. However, in this case it is desired to obtain the greatest possible output of modulated radio frequency energy. It is also preferable to operate the oscillator V5 so that its output is modulated 100%, i. e., its average plate current should vary from zero to a maximum determined by the tube and circuit characteristics, this variation of plate current following the audio frequency variations of voltage impressed on the grid of the modulator tube Vs.

The plate current of the modulator Vs cannot be varied from zero to a certain maximum value without very serious distortion; therefore, its plate current variations must be limited to a section of its characteristic curve which is practically straight so that appreciable distortion will not occur. To accomplish these conditions in the oscillator and modulator tubes, the value of resistance W is so adjusted that the resistance carries at all times the minimum modulator plate current. With this condition, when the plate current of the modulator tube is at its minimum value, the plate current of the oscillator will be zero. This results in 100% modulation. It will be seen that the plate current of the modulator is greater than the plate current of the oscillator by an amount equal to the amount of current flowing through resistance W. As in Fig. 1, the grid of the oscillator tube is connected, through a radio frequency choke coil to a point of fixed potential in the plate battery.

It should be noted that the tubes previously referred to as oscillators do not necessarily have to supply the radio frequency energy through self generated oscillations. These tubes may be only amplifiers of oscillations generated by a separate radio frequency generator. It is obvious that the modulation system described may be applied to either vacuum tube radio frequency oscillators or radio frequency amplifiers as it is merely a system of varying the plate current of the oscillator or amplifier at audio frequencies.

It will be understood that the above description and accompanying drawing comprehend only the general and preferred embodiment of this invention, and that various changes in construction, proportion and arrangement of parts may be made within the scope of the appended claims, and without sacrificing any of the advantages of this invention.

The herein described invention may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to me of any royalties thereon.

Having described this claim is:

invention, what I 1. In combination, a first electron device comohmic resistance means connecting the anode of said second device with its cathode, individual means for causing said cathodes to emit electrons, a common source of anode potential for energizing the anodes of both of said devices and means connecting the control electrode of said second electron device to a point of intermediate fixed potential in said common source of anode potential.

2. In combination a vacuum tube radio frequency oscillator, a vacuum tube audio frequency modulator, means connecting the anode of the modulator tube with the cathode of the oscillator tube, ohmic resistance means connecting the anodes of both of said tubes, individual means causing the cathodes of said tubes to emit electrons, a common source of anode potential for energizing the anodes of both of said tubes, and with the control electrode of the oscillator tube connected to a point of intermediate fixed potential in said common source of anode potential.

3. In combination a vacuum tube radio frequency oscillator, a vacuum tube modulator, means connecting the anode circuits of said oscillator and modulator in series, an ohmic resistance connected in parallel with the anode circuit of said oscillator, a source of anode potential common to the oscillator and modulator, the control element of the oscillator tube being connected through an oscillation circuit to a point of intermediate potential on said source of anode potential.

4. A vacuum tube audio frequency modulator, an ohmic resistance connected in series with the output circuit of said modulator, a source of power for said modulator, a vacuum tube oscillator with its anode circuit connected across said ohmic resistance, the control element of said oscillator being connected to a point of intermediate fixed potential in said source of power.

BURTON G. LAKE. 

